I'm pretty sure there's no showers at KOA unless you want to sponge yourself off using the sinks in the bathrooms at KOA. Aside from the Flagship lounge in LAX that you should be able to get a shower in, you can also get a day pass at planet fitness and use their showers. Alternatively, you're on the big island, go for a quick dip in the ocean after your hike and use the showers at the beach to get the salt water off and then change.